The present invention relates to a polysulfide polyether and a method of producing a polysulfide polyether and particularly to a polysulfide polyether having compatibility with plasticizers and containing only small amounts of low-molecular weight components and a method of producing such a polysulfide polyether. It further relates to a curable composition suitable for sealing materials needing only small amounts of plasticizers, which suffers from little transfer of plasticizers into a paint coating applied onto a cured product thereof, thereby avoiding the softening of the paint coating. Further, it relates to a polymer composition serving as curing agents for epoxy resins having a high curing speed and capable of providing a highly durable cured product.
When polymers containing two or more thiol groups in one molecule are mixed with an oxidizing agent, they are easily cured, and their thiol groups are easily reacted with epoxy groups, isocyanate groups, etc. to become high-molecular compounds. Accordingly, such polymers are widely used for sealing materials, paints, adhesives, etc.
Such a thiol group-containing polymer is described, for instance, in U.S. Pat. No. 2,466,963. Since the polysulfide polymer described in U.S. Pat. No. 2,466,963 has excellent oil resistance, chemical resistance, flexibility, etc., it is used as starting materials for sealants. When used with epoxy resins having two or more epoxy groups in one molecule, the polysulfide polymer can provide the epoxy resins with flexibility.
Since this polysulfide polymer has a structure of polysulfide bonds S.sub.x (x=1-5) in a molecule, it shows high polarity, and only limited types of plasticizers can be used. Since widely used, inexpensive plasticizers cannot be used, compounds such as sealants containing the polysulfide polymer are relatively expensive. In addition, since it shows a higher viscosity than expected from its molecular weight, a larger amount of a plasticizer is required to be added to improve the handling of the compound at a curing temperature. Accordingly, when a cured product of a sealant produced from the polysulfide polymer is covered by a paint coating, plasticizers tend to ooze out, thereby softening the paint coating.
Japanese Patent Publication No. 56-15751 discloses a curable composition comprising a polysulfide polymer and a polyester of mercaptocarboxylic acid and poly (oxyalkylene) polyol. Since the polyester in this composition does not contain disulfide bonds, it does not have good compatibility with the polysulfide polymer. Accordingly, only the conventional plasticizers compatible with polysulfide polymers can be used.
Apart from the above, the polysulfide polymer shows good adhesion strength and water-resistant adhesion, but is slow in curing at a low temperature when used together with epoxy resins.
As a thiol group-containing polyether having an improved low-temperature curing speed, Japanese Patent Publication No. 47-48279 discloses a liquid polymer represented by the general formula: ##STR1## wherein R is an alkylene group having 2-4 carbon atoms, and n is an integer of 2-6, and having a viscosity of 100-120,000 centipoise and a chemical equivalent of 150 determined by the thiol content. However, this polymer is poor in water-resistant adhesion, though it shows a good low-temperature curing speed.
In addition, when a polysulfide polymer and its composition with a thiol group-containing polyether are cured by an oxidizing agent, they become rigid, losing elasticity and durability when exposed to a high temperature of 80.degree. C. or higher.
As an oxidizing agent for polymers containing one or more thiol groups in one molecule, inorganic oxides, inorganic peroxides, organic peroxides, etc. are known, and metal peroxides such as PbO.sub.2, MnO.sub.2, ZnO.sub.2, CaO.sub.2, etc. are widely used.
A mixture of a polysulfide polymer and a thiol group-containing polyether is also easily cured when mixed with an oxidizing agent for the thiol group-containing polymers.
With respect to a mixture of a polysulfide polymer and a thiol group-containing polyether, sulfur is often added to improve its properties, but the addition of sulfur makes a curing speed of the mixture extremely slow.